Auto merge of #288 - posborne:fix-libc-export, r=fiveop
Auto merge of #288 - posborne:fix-libc-export, r=fiveop

libc: re-export libc properly

With rust 1.7, the following warning was being emitted by the compiler:

    warning: `pub extern crate` does not work as expected and should
    not be used. Likely to become an error. Prefer `extern crate` and
    `pub use`.

Based on https://github.com/rust-lang/rust/issues/26775 it appears that
this change is the current best approach for properly exporting the
libc dependency so it may be used outside of nix.

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

xd009642/nix,berkowski/nix,MarkusJais/nix-rust,carllerche/nix-rust,nix-rust/nix,posborne/rust-nix,posborne/rust-nix,xd009642/nix,Susurrus/nix,berkowski/nix,nikklassen/nix-rust,posborne/rust-nix,kamalmarhubi/nix-rust,asomers/nix,xd009642/nix,kamalmarhubi/nix-rust,carllerche/nix-rust,nikklassen/nix-rust,nix-rust/nix,geofft/nix-rust,berkowski/nix,Susurrus/nix,MarkusJais/nix-rust,kamalmarhubi/nix-rust,Susurrus/nix,nikklassen/nix-rust,MarkusJais/nix-rust,geofft/nix-rust,geofft/nix-rust,carllerche/nix-rust,asomers/nix